This invention relates to coolant systems for electric generators, and more particularly, to such systems for use in variable speed generators.
One type of aircraft electrical power generating system includes a generator which is coupled to an aircraft engine and driven at a variable speed. The AC output of the generator is rectified and feed to a pair of DC link conductors. An inverter receives voltage from the DC link conductors and produces a constant frequency AC output. A generator control unit senses voltage at some point in the system and adjusts the excitation current to the generator such that the generator output voltage is maintained within a predetermined range for various load conditions.
Certain electrical generators used in such variable speed constant frequency power systems utilize spray oil cooling. A coolant pump, which may be contained within the generator housing, operates at speeds which are proportional to the input speeds of the generator. This pump is driven either directly by the generator rotor shaft or through reduction gearing. Typically, gear rotor or vane type positive displacement pumps are used. These types of pumps have a power drain which is proportional to the product of rotational speed and back pressure of the coolant. The back pressure rises with increasing flow through orifices feeding the rotor, and the flow increases with speed. A pressure relief valve is required to reduce pump back pressure as speed increases.
The kinetic energy of the oil fed to the rotor also increases with speed and is lost when the oil is released to the generator case. This results in increased kinetic energy loss. Windage and churning losses also increase with speed, as does the erosion of insulation. Since load excitation losses, which are greatest in the rotating field, decrease as generator speed increases, the increased coolant flow and associated losses are wasted. It is therefore desirable to design a coolant system for a variable speed generator which can supply sufficient coolant at various operating speeds, while at the same time, minimizing losses over the operating speed range.